Motion compensated crown block system

ABSTRACT

A motion compensated crown block system in which a crown block sheave means is movable along a vertical path defined by a framework usually forming part of a drilling rig which is supported on a vessel. The crown block sheave assembly, which carries the hook load, is principally supported by inclined fluid pressure cylinder and piston means having one of their ends pivotally connected to the sheave means and their other ends pivotally connected to the framework in lateral spaced relation to the generally vertical pathway of the sheave means. Changing inclination of the cylinder and piston means during relative movement of the crown block sheave assembly and vessel provides variation in the piston reaction vertical force component which is caused to be nearly proportional to the change in fluid pressure in the cylinder and piston means. Load variation of less than 5 percent of the hook load is achievable over the full path of travel of the sheave means and is in the order of 2 percent over a major portion of said path in which the crown block sheave assembly travels. Means for sensing motion of the crown block sheave means relative to the sea bed are also provided so that the crown block sheave means will be virtually stabilized to maintain unchanged relation with respect to the sea bed during heaving of the vessel as caused by wind and wave action. Means are also provided for reducing to a minimum travel of the sheave cables over idle sheaves during travel of the crown block sheave means in its vertical path.

United States Patent [1 1 Burns et a1.

[ MOTION COMPENSATED CROWN BLOCK SYSTEM [75] Inventors: Jerome Q. Burns,La Mesa; William C. Green; Jack I. McLelland, both of Palos Verdes, allof Calif.

[73] Assignee: Ocean Science and Engineering,

Inc., Long Beach, Calif.

22 Filed: July 26,1972

21 Appl No.:27S,354

[52] US. Cl. 254/172, 91/422 [51] Int. Cl B66d 1/48 [58] Field ofSearch. 254/172, 173 R, 190 B; 175/5,

[56] 1 References Cited Primary Examiner-Allen N. Knowles Attorney,Agent, or FirmEdward F. Jams [57] ABSTRACT A motion compensated crownblock system in which a Feb. 12, 1974 crown block sheave means ismovable along a vertical path defined by a framework usually formingpart of a drilling rig which is supported on a vessel. The crown blocksheave assembly, which carries the hook load, is principally supportedby inclined fluid pressure cylinder and piston means having one of theirends pivotally connected to the sheave means and their other endspivotally connected to the framework in lateral spaced relation to thegenerally vertical pathway of the sheave means. Changing inclination ofthe cylinder and piston means during relative movement of the crownblock sheave assembly and vessel provides variation in the pistonreaction vertical force component which is caused to be nearlyproportional to the change in fluid pressure in the cylinder and pistonmeans. Load variation of less than 5 percent of the hook load isachievable over the full path of travel of the sheave means and is inthe order of 2 percent over a major portion of said path in which thecrown block sheave assembly travels. Means for sensing motion of thecrown block sheave means relative to the sea bed are also provided sothat the crown block sheave means will be virtually stabilized tomaintain unchanged relation with respect to the sea bed during heavingof the vessel as caused by wind and wave action. Means are also providedfor reducing to a minimum travel of the sheave cables over idle sheavesduring travel of the crown block sheave means in its vertical path.

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98% NOMINAL vac-KIN RANGE MOTION COMPENSATED CROWN BLOCK SYSTEMBACKGROUND OF INVENTION Drilling oil wells at subsea locations from afloating vessel presents difficult problems in maintaining a controlledposition of the drilling string relative to the sea floor to provideuniform stable drilling bit pressure and substantially uniform tensionin a drilling string because of the heaving motion of the vesselsupporting a drilling rig to which the drill string is attached. It isdesirable that the drill string be maintained at uniform tension andthat variations in tension be minimized in order to carry on normaldrilling and well completion operations, prevent undue stressing of thedrill string, uneven drill bit pressure, and excessive wear on thedrilling equipment, as well as to maintain a fixed inhole drill stringelevation for landing casing, tubing, setting packers, cementing,reaming and other operations requiring close elevation control.

Prior proposed systems of stabilizing or minimizing the relative motionbetween the vessel and the drilling string have included the use ofbumper subs, compensating the line tension of crown block and travelingblock sheave assemblies, compensating the traveling block sheaveassembly, and compensating the crown block sheave assembly. Variouscompensating systems are shown in Kammerer U.S. Pat. Nos. 2,945,677,3,151,686, 3,158,206, and 3,l58,208. A compensating system positionedbetween the traveling block and swivel of a drill string is disclosed inParks U.S. Pat. No. 3,208,728. Another type of compensating system isdisclosed in Berne et al. U.S. Pat. No. 3,285,574 wherein pneumatic oroleopneumatic vertical jacks and jacks inclined to the path of a returnpulley means over which passes a flexible conduit or tubing used inplace of a drill string for turbine drilling.

A prior proposed motion compensated crown block is disclosed in HortonU.S. Pat. No. 3,469,820, owned by a common assignee, in which a systemof pneumatic cylinder and piston means support a major portion of theload on a crown block sheave assembly and double acting hydrauliccylinder and piston means are employed to additionally support suchload.

In such prior proposed systems mentioned above and in which pneumaticsystems are employed to support the load, the volume of gas required inthe pressure system is very great and space requirements for gasaccumulators are very substantial. In addition, such prior pneumaticsystems have used costly inert gases, such as nitrogen, since use oflarger volumes of air at required high pressures was hazardous anddangerous with oleo systems. Moreover, the force-displacementrelationship to accomplish the desired motion compensation by the use ofprior proposed gas and hydraulic cylinder systems permitted motioncompensation to be achieved at percent or more of the total weight ofthe supported load including frictional loads. Thus while variation ofdrill bit pressure was previously held within a relatively narrow range,the amplification and enlargement of equipment to still further narrowthe load limits was not justified by the additional material costs andspace involved.

In addition, the vertical movement of the drill string causes theshifting of stresses in the drill collars where the point of neutralstress (change between compression and tension) may cross a drill collaror pipe joint causing undue stressing of such joint and possiblefailure. Such constantly changing stress at drill collar sections maycause deformation of the drill collars and produce unnecessary cuttingof sides of a hole or hole deviation by the resultant dissipation ofdrilling energy into a lateral direction instead of a verticaldirection.

SUMMARY OF INVENTION The present invention contemplates a novel crownblock motion compensating system particularly useful in performing welloperations at sea including drillings, well completion, work-over andany other operation wherein motion of a floating installation must becompensated to provide close elevational control between elements fixedwith respect to a reference point such as the sea bed and an elementmovable relative thereto. The present motion compensating system affordsoperational and economic advantages over prior proposed systems whileavoiding their disadvantages. Generally speaking, the motioncompensating crown block system of the present invention provides aplurality of fluid actuated cylinder and piston means so arranged withrespect to a crown block sheave means as to vary the sheave supportingvertical force component exerted by the fluid cylinder and piston meanson the sheave means during travel of the sheave means in a verticalpath. As a result of this disposition of the fluid primary supportcylinder and piston means and the correlation therewith of a selectedgas pressure and accumulator volume for the cylinder and piston means, arelatively flat force-displacement curve (hook load change versus crownblock vertical displacement relative to the vessel) is obtained andaccumulatorvolume is minimized. The accumulator system of a volumecapable of use with this invention may be mounted on top of a derrickimmediately adjacent the crown block sheave assembly, thereby avoidinglong lines and attendant frictional losses and maintenance. Theinvention further contemplates an idler sheave arrangement associatedwith the crown block sheave means which will reduce wear (ton mileage)on the wire rope or cable used in the sheave means. The inventioncontemplates a sensing means for the heave motion of the vessel so thatpressure in the fluid cylinders may be manually or automaticallycontrolled in order to maintain stability of the crown block sheavemeans with respect to the sea bed, and thereby the drilling string withvirtually uniform tension therein.

It is therefore a primary object of the present invention to discloseand provide a novel motion compensating system for a crown block sheavemeans used in various well operations as well as transfer and liftingoperations performed at sea, the sheave means being carried by a vesselsubject to wave motion.

An object of the present invention is to disclose and provide aresilient highly responsive support means for a crown block sheave meanson a drilling rig wherein the arrangement of the resilient support meansand its connection to the sheave means and the drilling rig frameworkenhances the achievement of a desirable hook load versus crownblockdisplacement curve.

Another object of the present invention is to disclose and provide amotion compensated crown block sheave system wherein a plurality offluid cylinder and piston means are especially arranged with respect tothe crown block sheave means to permit reduction of accumulator volumespace to a minimum.

A further object of the present invention is to disclose and provide anovel motion compensated crown block sheave system wherein idler sheavesare so arranged and supported with respect to the crown block sheavemeans that wear on the sheave lines is minimized.

A still further object of the present invention is to disclose andprovide a crown block sheave system as mentioned above wherein theinboard and outboard path of movement of idle sheaves is related tomovement of the sheave means in its vertical path to minimize wear onthe sheave lines.

Still another object of the present invention is to disclose and providea means for sensing the effect of wave motion on the vessel andtransmitting such motion sensing to suitable means for controllingpressure exerted by the fluid inclined cylinder and piston means whichsupport the crown block sheave means.

A still further object of the present invention is to disclose a motioncompensating system for a sheave assembly utilizing inclined cylinderand piston means embodying fluid control means for cushioning movementof the sheave means at extremities of its normal path of travel andwherein a framework providing a slidable guide means for the sheavemeans is provided with shock or load absorbing means at the ends of thepath of movement of the sheave means.

A still further object of the present invention is to disclose andprovide main support cylinder and piston means constructed to functionas a combined fluid support for the sheave means and also as a dashpotmeans when the sheave means approach ends of its path of travel.

A still further object of the present invention is to disclose andprovide a crown block sheave means guidably supported for movement in apathway relative to heaving motion of the vessel and wherein shockabsorbing means are provided at opposite ends of the pathway of thecrown block sheave means.

A still further object of the present invention is to disclose andprovide a motion compensating system wherein main support cylinders andpiston means actuated by an air-liquid system are provided with safetyand protective means in the event unusual undesired loads are imposedupon the sheave means and the framework in which the sheave means isguided.

The invention further contemplates a motion compensating system for asheave assembly movable in a vertical pathway wherein total loadvariation is minimized during movement of the sheave assembly in itspathway, wherein a novel idler sheave linkage cooperates with the sheaveassembly to further reduce or eliminate the effect of changes in hookload, wherein fluid cylinder and piston means of hydraulic orhydropneumatic types are associated with said sheave asseembly to serveas dashpots therefor in the event of an unexpected. unwanted change inhook load, loss of air pressure in the primary support cylinder andpiston means, or other failure in the system; wherein the basic systemmay be readily made positively active to virtually eliminate drill bitload variation by maintaining a constant distance between the sea bedand the crown block sheave assembly, and wherein cost and time savingsare obtained by increased drilling efficiency and less equipmentdowntime.

Other objects and advantages of the present invention will be readilyapparent from the following description of the drawings in whichexemplary embodiments of the invention are shown.

In the drawings:

FIG. 1 is a fragmentary schematic view of a drilling rig on a floatingvessel provided with a crown block sheave means embodying thisinvention.

FIG. 2 is a schematic view of the crown block and traveling block sheavemeans shown in FIG. 1.

FIG. 3 is an enlarged fragmentary elevational view of the top of adrilling rig equipped with a crown block sheave system as shown in FIG.1, the sheave means being at the lower end of its vertical path, andcorresponds to the position of the vessel at the top of a wave crest.

FIG. 4 is a fragmentary elevational view of the sheave system shown inFIG. 3 wherein the crown block sheave means is at a midpoint of itsvertical path of travel, and corresponds to a vessel at the midpoint ofits heave motion.

FIG. 5 is a fragmentary elevational view of the crown block sheavesystem shown. in FIG. 3 wherein the sheave means is in its uppermostposition in its vertical travel, and corresponds to the position of avessel at the lowermost point in a wave trough.

FIG. 6 is an enlarged fragmentary sectional view taken from thehorizontal plane indicated by line VI-VI of FIG. 5.

FIG. 7 is a fragmentary elevational view of a modification of the crownblock motion compensating system of this invention.

FIG. 8 is a further modification of the crown block motion compensatingsystem of this invention.

FIG. 9 is a schematic elevational view, partly in section, of a motionsensing means utilized in the present invention.

FIG. 10 is a fragmentary elevational view, partly in section, of amodification of such motion sensing means shown in FIG. 9.

FIG. 10a is a fragmentary schematic view of a still further modificationof a motion sensing system.

FIG. 11 is a schematic view of a pneumatic pressure system utilized inthe present invention.

FIG. 12 is a schematic view of a combined gas-liquid pressure systemadapted for use with the motion compensating system of this invention.

FIG. 13 is a graph showing exemplary hook load crown block displacementcurves, the abscissa showing length of stroke of the crown block, theordinate indicating percent of hook load.

FIG. 14 is a schematic view of a further modification of inclined fluidcylinder and piston means utilized to support the sheave means andembodying a novel builtin dashpot system.

FIG. 15 is a schematic view with fragmentary cylinder and piston meanspartly in sectionand showing the combined dashpot andsupport system ofFIG. 14.

FIG. 16 is a diagrammatic layout of an exemplary hydropneumaticactuating system used in the compensating system of this invention.

In FIG. 1 a floating vessel 20 is fragmentarily shown and may carry adrilling derrick or rig 21 constructed in well-known manner. At the topof derrick 21 a superstructure comprising a framework means 22 isprovided. Framework means 22 includes a water table or platform 23 andan upstanding framework 24 which defines a vertical pathway for movementof a crown block sheave means 25. Crown block sheave means is reavedwith a wire rope 26 to a traveling block 27 which carries attachmentmeans 28 for connecting the traveling block to the upper end of a drillstring 29. The traveling block is prevented from rotation about itsvertical axis by sidewardly extending arms 30 which may slidably engagespaced parallel vertical guide lines 31. Drill string 29 extends belowvessel 20 through the water of the ocean to a well hole being drilled inthe sea bed, not shown. At the bottom end of the drill string is a drillbit, not shown, upon which a desired predetermined pressure is imposedto provide optimum drilling. The load thus imposed upon the crown blocksheave means 25 principally includes the weight of the drill string lessthe weight of the drill bit. In FIG. 2 the line 26 is passed over idlersheaves 32 supported in novel manner laterally of the axis of the crownblock sheave means. The dead line portion 33 of the line 26 may be fixedto the vessel and the fast line portion 34 may be connected to drawworks drum 35 suitably driven to take in and pay out line 34 as thelength of the drill string is increased or decreased during drillingoperations, (not motion compensating operations).

In the embodiment of the invention shown in FIGS. 1-6 inclusive, thesuperstructure framework means 22 at the top of derrick 21 includes ahorizontal platform 23 from which the upstanding framework 24 extendsand provides means for guiding said sheave means in said verticalpathway. Framework 24 may comprise a rectilinear arrangement of spacedsquare section vertical guide columns providing opposed longitudinalside faces 41 engaged by rollers 42 (FIG. 6) carried on end plate means43 of crown block sheave means 25. Inboard faces 44 of each column 40provide guide surfaces for rollers 45 carried on side plates 46 ofsheave means 25. The square section columns 40 thus provide verticalguide faces 41 and 44 generally perpendicular to each other andengagement thereof by rollers 42 and 45 guides vertical movement ofsheave means 25 without askew or lateral displacement of the sheavemeans. The guide columns 40 may be strengthened and braced by inclinedcolumns 48. Upper ends of columns 40 and 48 may be interconnected by tophorizontal transverse members 49. Crown block sheave means 25 is therebyguidably movable in a vertical path between lowermost and uppermostpositions as shown in FIGS. 3 and 5 respectively.

End plates 43 of the sheave means 25 support in bearing means 50 a crownblock sheave shaft 51 which may extend beyond each of the end plates 43.A plurality of sheave members are carried by shaft 51 between end plates43. Immediately below the crown block sheave means 25 traveling blockmeans 27 also comprises a sheave shaft provided with a plurality ofsheave members carried thereon. Line 26 may be reaved in wellknownmanner about sheave members of the crown block and travelling block.

Means for resiliently supporting the crown block sheave means 25 in thisexample, comprises sets of fluid actuated cylinder and piston means 55at opposite ends of crown block sheave means 25. Each cylinder andpiston means 55 is provided at its upper end with a pivotal connection560 to pivot shaft 56b which extends between and through end plates 43(FIGS. 4 and 6). A pivotal connection 57 to framework means 22 isprovided at the opposite end of each cylinder and piston means 55 inlateral spaced relation to the pathway of the sheave means and below thelower most position of the sheave means. In the lowermost position ofcrown block sheave means 25 (FIG. 3) the inclination of the axis of thecylinder and piston means 55 lies at a selected angle theta 0 to thevertical path of the crown block sheave means. A projection of saidpiston means axis may pass slightly below the axis of the sheave shaft51. The angle theta of inclination of piston means 55 increases assheave assembly 25 moves vertically upward to its uppermost position(FIG. 5). Vertical displacement of the sheave assembly 25 from lowermostto uppermost position may be in the order of from 1 to 30 feetdedepending upon design parameters.

As best seen in FIGS. 3 to 5 inclusive, piston rods 58 of cylinder andpiston means 55 are projected and inclined upwardly during verticalmovement, of the crown block sheave means. As schematically illustratedin FIG. 11, the magnitude of the vertical force component is a functionof the sine of angle theta and may be expressed as Fv=Fc sin thetawherein Fv is the vertical force component, Fe is the resultant forceexerted by the cylinder and piston means 25, and angle 0 is the angle ofinclination of the axis of the cylinder and piston means 55 with respect to the horizontal. Thus a variable vertical force component isprovided which is related to the varying angle of inclination of thecylinder and piston means 55 and to the position of the sheave means inits vertical pathway as later more fully described with relation to theconstruction and operation of the cylinder and piston means 55.

In one example of this invention, cylinder and piston means 55 maycomprise fluid actuated means such as air or gas as shown in FIG. 11.Piston rod 58 may comprise a hollow tube 61 having an outer diameterslightly less than the inner diameter of cylinder member 62 to providean annular space 63 therebetween of relatively small volume connected byports 61a while providing a hollow inner chamber 64 within piston tube61 of relatively large volume. A piston head 65 is provided at the innerend of piston tube 61 to slidably engage the internal surfaces of thecylinder member 62. At the lower end of cylinder member 62, a pressurefluid line 66 may be suitably connected at 67 in fluid communicationwith the interior of cylinder member 62. Pres sure fluid line 66 isconnected at its other end to an accumulator or pressure vessel 69. Avalve is provided in pressure fluid line 66 for control of pressurefluid between the accumulator and the cylindrical member 62.Accumulators 69 may be connected to a suitable source of pressure fluidsuch as a reservoir and air compressor, not shown.

Each accumulator 69 in this example may be located on or near platform23 and is positionedclose to the cylinder and piston means 55 with whichit is associated. To provide uniformity of pressure in the two sets ofcylinder and piston means 55 located at opposite ends of the crown blocksheave assembly, the accumulators may be interconnected by suitablelines for equalization of pressure therein. Accumulators 69 areillustrated as a spherical pressure vessel; other suitable pressurevessel shapes may be used. Other arrangements of accumulator 69 may beprovided depending upon available space at the superstructure framework,for example, accumulators 69 may be stacked vertically one upon theother, accumulators 69 may be arranged in a row, or arranged and locatedin other suitable fashion.

It should be noted that accumulators 69 are relatively small and thatthe total accumulator volume includes not only that of the pressurevessels 69 but also the volume of the chambers in the cylinder member 62and the diametrically enlarged piston tube 61. Pressure in cylinder andpiston means 55 is controlled to produce a resultant force which has avertical force component of a magnitude sufficient to support the hookload less the weight on the drill bit and with the crown block sheaveassembly at a midportion of its vertical stroke. As the vessel movesrelative to the crown block sheave assembly because of wave action, theincrease or decrease in pressure in the cylinder and piston means 55balances the variation in vertical force component caused by thedifference in inclination of the cylinder and piston means 55 asindicated by the change in angle theta so that variation in the hookload caused by heaving of the vessel is minimized as more fullydescribed later.

The crown block sheave means 25 may also be connected to a dashpot means72, in this example, comprising a vertically disposed hydraulic cylinderand piston means 73 having a piston rod 74 pivotally connected at 75 toend plate means 43 of the crown block sheave assembly. The lower end ofcylinder means 73 may be connected by trunnions 76 to frameworkstructure adjacent to platform 23. The dashpot means 72 may include adouble-acting fluid arrangement utilizing a liquid or an oil wherein thefluid may flow freely between opposite sides of the piston head thereinby the provision of suitable passageways. When the crown block sheaveassembly reaches the end portion of its vertical pathway, thepassageways in the dashpot means are so arranged that the motion of thecrown block sheave assembly is rapidly dampened and controlled so thatover-running of the crown block sheave means caused by some failure inthe system will be controlled and the motion cushioned so thatsubstantial damage to the structure may be avoided. Thus in the event offailure of the main pressure fluid support system, the dashpot means 72will cushion the movement of the crown block sheave assembly into itslowermost position in its pathway and in such position the crown blocksheave assembly may be utilized without motion compensation advantages.Dashpot means 72 may also provide a means for positive, activeadjustment of the position of the crown block sheave assembly duringoperations which require precise vertical positioning thereof. It mayalso be noted that the hydraulic system for the dashpot means isseparate from the pneumatic air system for primary support cylinder andpiston means 55.

The present invention contemplates a novel arrangement for reducing wearor ton mileage on the sheave line 26. Each idler sheave 32 is supportedfor movement toward and away from the vertical pathway of the sheaveassembly 25 by a support member 80 having a pivotal connection at itsupper end to the idle shaft 81 of idler sheave 32 and having a pivotalconnection at its lower end at 82 to a bracket 83 on platform 23. Idlersheave 32 is supported for arcuate movement relative to the pivotalconnection at 82.

Lateral in and out motion of each idler sheave 32 with respect to thevertical pathway of the crown block sheave assembly 25 is controlled bya link member 85 pivotally connected at one end to idler sheave shaft 81and pivotally connected at its other end to shaft 51 of the crown blocksheave assembly. Thus as the crown block sheave assembly moves up anddown (FIGS. 3-5) the length L of the cable portion extending betweentangent points on idler sheaves 32 and the crown block sheaves remainsunchanged. Thus linear motion of line 26 relative to the idle sheave isvirtually eliminated because the line 26 merely partly wraps or unwrapsfrom the circumference of the sheaves. Such elimination of relativelinear motion of line 26 at sheave 32 essentially eliminates theton-mileage on the drill line, reduces to a minimum wear or fatigue andduring the stroke of the crown blocksheave means, distributes vibrationfatigue over the portion of a line which is being wrapped or unwrappedon the idler sheave 32.

When sheave means 25 is in its lowermost position, idler sheaves 32 aredrawn inwardly toward the vertical pathway of the sheave means (FIG. 3).When the sheave means 25 is at the midportion of its vertical pathway,the idler sheaves 32 are spaced their maximum distance from the verticalpathway as determined by the length of the link members 85. At theuppermost position of sheave means 25, the idler sheaves 32 are againdrawn inwardly toward the vertical pathway. Thus during heave motion ofthe vessel, the line 26 is alternately unwrapped from the crown sheavesand wrapped on the idler sheaves without any change in length of theline portion extending between tangent points on the sheaves. It will beunderstood that the link member 85 does not change normal stretching ofline 26 that may occur during hook load changes.

Means for sensing vertical motion of the vessel relative to a fixedreference such as the sea bed is shown in FIGS. 9, 10 and 10a. In theembodiment shown in FIG. 9, a tension line becketed at the water tableis passed over a sheave at 91 mounted on the crown block sheave assembly25. Line 90 is passed over idle pulleys 92 in any suitable location onthe drilling rig 22 to provide a most direct, non-obstructed pathway toa drill opening 93 in the vessel through which line 90 extends forconnection to a pulley 94 pivotally mounted on an anchor base 95 on thesea floor 96. Line 90 may be returned to a deck 97 on the vessel forconnection to a tensioning means 98. Tensioning means 98 may include anair or hydraulic operated take-up reel which may be preset to a selectedtension in line 90. A fixed length between the crown block sheaveassembly and the sea bed is thus determined. The tension in line 90varies as the vessel heaves. Such change may be sensed by a suitabletension sensing means 100 in engagement with a portion of the line 90above the deck 97. Tension sensing means 100 includes motion sensing ofthe crown block sheave assembly 25 and such motion may be indicated upona suitable readout instrument 101 provided at a control console used bya drilling operator.

Another example of the vertical motion sensing system is shown in FIG.10 wherein line 90' is connected at 91' to the end plate means 43 of thecrown block sheave assembly 25. Line 90 is also guided over idle pulleys92, its tension may be sensed by a tension sensing means 100, and itslower end may be connected at 103 to a tension line 104 used with ariser pipe tensioning system indicated at 105. Riser tension line 104may be suitably connected at 106 to riser pipe 107 which extendsupwardly from the sub sea well head 108. A slip joint 109 at the top ofriserpipe is provided for vessel heave and tide conditions. To controlthe length of line 90' and its tension during start up of operation ortidal changes, a tensioning means 98 is inserted in the line 90'.Tensioning means 98' may be a double drum winch with a differentialdrive and controls wherein one drum is connected to the upper lineportion and the other drum to the lower line portion. The drums may havea common axis, the diameter of the drums being identical. It will beapparent that connection of line 90 to the riser pipe tension line 104,or riser pipe 107 is essentially the same as connecting line 90 to thesea floor in that a fixed reference is provided at a fixed distance fromthe sea floor.

In FIG. a a further modification of the vertical motion sensing systemis schematically shown. Line 90" may be connected at its upper end tothe crown block sheave means, as in the prior embodiments, and at itslower end to a guide line 110 which is used in the normal guide linesystem for lowering equipment to anchor base 95, the other guide lineused in such systems not being shown for clarity. The connection of line90 to guide line 110 may be made at any suitable location such as at111. It will be apparent that the connection at 111 provides anessentially fixed reference point with respect to the sea bed. Operationof this modification is similar to the operation of the motion sensingmeans described with respect to FIG. 10.

Before describing operation of the motion compensated crown block meansof this invention, a description of the modifications shown in FIGS. 7and 8 will be made. In FIG. 7 a modification of the means for mountingthe cylinder and piston means 55 on the framework means is illustrated.Cylinder and piston means 55' are inclined to the vertical pathway ofthe crown block sheave means 25' in accordance with the invention. Meansfor pivotally mounting each cylinder and piston means 55 may comprise amounting member 112 at the central portion of the cylinder member 62',member 112 being pivoted at 113 to a framework bracket member 114depending from platform 23 and at a pivot axis spaced from the verticalpathway of the sheave means 25 and below the lowermost position of thesheave means in its stroke. Thus as the sheave means 25 travels upwardlyin the vertical guide means, the cylinder member 62' is permitted somerotation about the pivot axis 113. The upper end of the piston member 58is connected to the outer end of the sheave shaft 51 so that the axis ofthe cylinder and piston means 55 approximately passes through the axisof the sheave shaft 51 during travel of the sheave means 25' in itsvertical pathway.

Framework bracket member 114 may also support a holding means 116 towhich may be fixed the cylinder member 73 of the dashpot means 72.

It will thus be understood from the consideration of FIG. 7 that thecylinder and piston means 55 and the dashpot means 72' may be mounted onthe framework means 23 in various ways so as to permit variation of theangle theta of the axis of the cylinder and piston means 55 during thetravel of the crown block sheave means and also to afford a desiredmagnitude of vertical force component acting to resiliently support thecrown block sheave means 25.

In FIG. 8 there is shown a modification of the resilient support meansfor the crown block sheave means and also a modification of the supportfor the idle sheave means.

Resilient support means shown in FIG. 8 may include cylinder and pistonmeans 55" pivotally connected at 120 to platform 23 and pivotallyconnected at 121 to a triangular end plate means 122 at one of thecorners of said triangular plate. Crown block sheave means 25 mayinclude a sheave shaft 51" supported on end wall 124 of a verticallyslidably movable carriage 125. Carriage 125 may be guided in itsvertical pathway by a plurality of upstanding guide members 126 suitablybraced and defining the vertical pathway for carriage 125. Carriage 125may be provided with suitable antifriction means engaging upstandingguide means 126 in well-known manner. Triangular arrangement on plate122 is disposed in inverted fashion and the lower corner of the plate ispivotally mounted at the axis of shaft 51".

Cylinder and piston means 55" may operate as described in the firstembodiment of the invention. In this modification, however, auxiliarycylinder and piston means 128 may be provided with the piston rod 129pivotally connected at 121 to the corner of the triangular plate 122 atwhich a rod 58" of the cylinder and piston means 55" is also connected.The opposite end of cylinder and piston means 128 may be pivotallyconnected at 130 to an outwardly extending member 131 fixed toupstanding guide members 126 at a selected portion of the vertical pathdefined thereby, in this example, above the central portion of thepathway. In such mounting of the auxiliary cylinder and piston means128, it will be apparent that when the crown block sheave means 25" isat the lowermost position of its path of travel, the cylinder and pistonmeans 128 are inclined downwardly and inwardly. At the uppermostposition of the crown block sheave means 25", the auxiliary cylinder andpiston means 128 is inclined upwardly.

Cylinder and piston means 128 may be single or double-acting. As thecrown block sheave assembly means progresses to its lower-most position,the auxiliary cylinder and piston means 128" exerts a downward verticalforce component against sheave assembly means 25". This force componentcompensates the increasing force developed in the main cylinder 55"because of the decreasing volume in the accumulator system. It will beapparent that when the sheave means 25" is at a midportion of itstravel, in view of the height of plate 122, that the auxiliary cylinderand piston means 128 will be substantially horizontal and no verticalforce component will be exerted on sheave means 25". As the sheave meansprogresses to its upper-most position, it will be apparent that theauxiliary cylinder and piston means 128 exerts an upward vertical forcecomponent which helps support the sheave assembly means 25" in a manneropposite to that described above with respect to movement to lower-mostposition. Thus the auxiliary cylinder and piston means 128 act as trimcylinders to more precisely provide the desired vertical force componentto maintain the crown block sheave assembly in stable relation to thefixed reference point or sea bed.

It may be noted that in the several embodiments shown that the pivotalconnections of the cylinder and piston means 55, 55 and 55" vary intheir distance from the axis of the vertical pathway of the sheave means25. The specific location of the fixed pivotal connection of the primarycylinder and piston means 55 and also of the auxiliary or trim cylinderand piston means 128 becomes quite critical and is dependent uponseveral factors which affect the optimum design, namely the expectedvariation in hook loads, that is the weight of the drilling bit anddrilling string, the drilling string changing in length as the well holeis drilled, the desired pressure at which the cylinder and piston meansmay be operated, and the total volume of the accumulators, such totalvolume being held to a minimum. Thus the difference in lateral andvertical spacing of the fixed pivotal connection of the cylinder andpiston means from the vertical pathway is dependent upon several factorsas mentioned above.

FIG. 8 also illustrates a modification of the support means forpermitting inward and outward movement of the sheave means 32 from thevertical pathway as the crown block sheave means reciprocally moves upand down therein. In this embodiment each idle sheave 32, is pivotallymounted on a carriage 135 having rollers or wheels 136 guidably movablealong a horizontal track 137 which extends outwardly from the verticalguide frame members 126. As in the previous embodiment, the pivotal axisof each idle sheave 32' is pivotally connected with one end of a linkmember 138 which has a pivotal connection at its other end to the sheaveshaft 51" at the lower corner of plate 122. Thus during the verticalmovement of sheave means 25" the length of the line portion 139 of theline 26 as measured between tangent points on the idle sheave and thecrown block sheave remains constant, and wear on the line 26 is reducedto a minimum by such inward and outward lateral movement of the idlesheaves 32.

In the above described embodiment of the invention the primary supportmeans for the crown block assembly included air or gas actuated cylinderand piston means. Under some conditions it is desirable to use gasliquidcylinder and piston means wherein the gas may be either air or someinert gas such as nitrogen and the liquid means may be an oil orpreferably a water base liquid. Two examples of air-liquid systems arehereafter described and are part of this invention.

In the first example of such an air-liquid system, the cylinder andpiston means supporting the crown block assembly may include theconstruction shown in FIG. 12 wherein each of cylinder and piston meansgenerally indicated at 145 may include a cylinder member 146 and apiston member 147 of solid construction provided with a piston head 148.Bypass passageways 149 may be provided in the solid piston member 147between opposite sides of the piston head 148. The upper end of pistonmember 147 may be pivotally connected to the crown block sheave assemblyas in the prior embodiment and the lower portion of cylinder member 146may be pivotally connected to the framework means as in the priorembodiment.

A suitable gas and liquid reservoir 150 may include a floating piston,bladder or membrane 151 at the interface between the gas and liquid toprevent entrapment of gas in the liquid during operation. The gasportion of the reservoir 150 may be connected by line 152 to accumulator69' as diagramatically indicated in FIG. 12. A control valve 153 isprovided in gas pressure line 152. The liquid portion of the gas-liquidmeans 150 is connected by a flow line 155 to the bottom portion of thecylinder member 146 and has a control valve 156 in line 155. Controlvalve 156 senses the flow of liquid through line 155 and at apreselected flow speed valve.

156 will close and thus lock out the piston and cylinder means toprevent travel of the crown block means beyond a certain point.

In a further embodiment of the present invention shown in FIGS. 14 and15 generally only those portions of the modified construction differingfrom the prior embodiment of the invention will be described forpurposes of clarity and brevity.

In FIG. 14 framework means provides a vertical pathway for a crown blocksheave means 171 in a manner similar to the prior embodiments. The crownblock sheave means 171 includes end plates 172 which supporttherebetween a sheave shaft 173 which carries the sheave 174, and whichsupport through shafts 175 which provide pivotal connection for upperends 176 of piston rods 177 of the four inclined cylinder and pistonmeans 178. Cylinder and piston means 178 are each provided a pivotalconnection at 179 to a frame member 180 forming part of the drilling rigsuperstructure.

Sheave means 171 may move in its path to the upper end 181 of theframework means 170 and to the lower end 182 thereof. Normally thestroke of .the sheave means 171 is limited to a portion of the pathwaybetween the extreme ends of the framework 181 and 182.

In this embodiment of the invention, another modification of means forcushioning and slowing down movement of the crown block sheave means atand adjacent to ends of its path of travel are provided. In FIG. 15 eachinclined cylinder and piston means 178 may comprise a cylinder member184 having a cylinder chamber 185 therein. Within chamber 185 isreciprocally slidable a piston head 186 carried on the end of piston rod177 provided with an internal chamber 187. A suitable seal means 188 isprovided between the cylindrical member 184 and the piston rod 177 atthe upper end of the cylindrical member 184.

Means are provided at piston head 186' for permitting flow of liquidfrom cylinder chamber 185 to annular portion 190 of chamber 185 on theupper side of piston 186 and from annular chamber portion 190 to theinternal chamber 187 of the piston rod 177. In this example, suchcommunication is provided by an axial port 191 in piston head 186 whichleads to a valve chamber 1 192 in which a ball valve element 193 isbiasedt'o normally open position by a suitable coil spring 194. Atapered ball valve seat 196 surrounds the inner endof port 191. Betweenport 191 and valve seat 196 a counterbore is provided to seat andcontain spring 194 when the ball valve is closed. In closed position,the top of ball 193 lies below the inlets of passages 198 leading toannular chamber portion 190. The walls of piston rod 177 are providedwith ports 199 for communication between annular chamber portion 190 andinner chamber 187 of the piston rod. When the ball valve 193 is closed,fluid in chamber 190 is virtually confined except for limited flow aboutthe piston head. A dashpot function is thus incorporated into theinclined cylinder and piston means and is operable in the event ofsudden loss of load on the cylinder and piston means.

In operation of the cylinder and piston means 178, cylindrical chamber185 may be connected through port 200 with a fluid or liquid line 202connected to a reservoir 203 containing a body of liquid fluid such as awater base liquid or a suitable oil. The upper portion of reservoir 203includes a chamber 204 for a compressable gaseous fluid such as airwhich may be connected by pressure line 205 to an accumulator and acompressor, (not shown). At the interface between the air and waterbased liquid may be a bladder or a suitable free floating follower orpiston 207 for preventing absorption of one fluid into the other toreduce corrosion, to prevent possible air saturation of the liquid, andto facilitate equalization of liquid flow to the several main inclinedsupport cylinders. The reservoir 203 is preferably located near the topof rig structure and close to the support cylinders to minimize liquidflow losses.

In fluid line 202 emergency shut off valves 210 and 211 are provided.Valve 210 may be of quick closing type and is operable in the event of asudden loss of fluid pressure in the hydropneumatic system which wouldresult in a failure of the support system for the sheave means. Thusupon sudden loss of pressure in the air reservoir the shut off valve 210would become operable to maintain sheave supporting pressure in thecylinders.

Valve 211 is located immediately adjacent each cylinder and piston means178 and becomes operable in the event of loss of all or a portion of thedrill string or if the drill pipe breaks. Under such conditions thesudden relief from load on the cylinder and piston means would cause thepistons to urge the sheave means upwardly against the top of theframework. Closing of shut off valve 211 stops flow of liquid into thecylinder and piston means so that in cooperation with the dashpotarrangement of the cylinder and piston means such reaction to a pipebreak is minimized.

In the event the crown block sheave means and end plates which are beingguided in the framework 170 reaches the extreme ends of their path oftravel at 181 or 182, shock absorbing means 214 at the top of theframework and shock absorbing means 215 at the bottom of the frameworkare provided to cushion and stop the travel of the sheave means. Shockabsorbing means 214 and 215 may comprise a selected number of shockabsorbing units which may be of well known construction and manufactureand may essentially comprise a cylinder with liquid fluid therein and apiston ported to control passage of fluid from one side of the piston tothe other so that upon contact of the unit by the sheave means 171motion of the sheave means will be cushioned and stopped withoutexcessive damage to the framework 170.

In operation of the crown block compensating means of this invention, itwill be understood that at the top of the drilling rig the crown blocksheave means is resiliently supported by inclined cylinder and pistonmeans which are connected to working accumulators relatively closelyadjacent to the associated cylinder and piston means and that a liquidactuated dashpot cylinder and piston means is arranged to freelycirculate liquid during compensation and hydraulic power means forpumping or supplying liquid to the dashpot means are also closelylocated with respect thereto. At the deck ofthe drilling rig or belowthe deck is provided an air compressor and an initial air supplyreservoir which generally comprises only a relatively small portion ofthe gas volume as for example about ll5 percent.

At the deck a control console is provided for the drilling operatorwhich may include the heave stroke posi-' tion indicator, a hook loadpressure gauge, a compensator lock out control button for control of theliquid lines, a pressure gauge for the air reservoir and an air pressurebleed off control button.

In FIG. 16 an exemplary operating or actuating system for the motioncompensating apparatus shown in FIGS. 14 and 15 is illustrated. It willbe understood that a generally similar actuating system may be employedwith respect to the apparatus of the prior embodiments utilizing anair-liquid actuating means In the prior embodiment utilizing only air,an actuating system similar to that described in Horton US. Pat. No.3,469,820 may be employed.

In FIG. 16 the load supported by the inclined hydraulic cylinders 178 isexemplary and schematically illustrated as 440,000 pounds which wouldrepresent the load carried by the crown block sheave means 171. Eachcylinder 178 is in communication through fluid conducting line 202 withthe hydropneumatic reservoir 203. Automatic cylinder emergency shut offvalves 211 are shown adjacent each cylinder 178 and each valve 211 isprovided communication with the other valves 211 through connectinglines 220. Adjacent reservoir 203, a quick closing safety valve 210 isprovided as above described. The air portion of reservoir 203 isconnected by line 205 with a supply accumulator 221 containing air underpressure, which is supplied by an air compressor 222 havingcommunication through line 223 with a make up air supply reservoir 224from which air under pressure is applied through an air controller 225to the accumulator for reservoir 203. Air controller 225 may comprise asuitable flow valve sys tem for maintaining a selected pressure inaccumulator 221 and reservoir 203. An air control system 226 isconnected by a suitable line 227 to air control 225. Air control system226 provides a means whereby an operator on the rig may change andregulate the pressure supplied to the reservoir in accordance with theconditions encountered during initial make-up and later changes inoperation.

Generally speaking, maintenance of a fixed distance between the crownblock sheave means and a fixed reference point such as the sea bed isdesired and in addition, hook load variation must be minimized in orderto provide optimum drilling bit weight. Therefore, as wave action causesthe vessel to heave, relative movement occurs between the crown blocksheave assembly and the framework means. Thus as the vessel movesupwardly, the crown block moves downwardly relative thereto and gas inthe cylinder and piston means is compressed. Likewise as the vesselmoves downwardly and the crown block moves upwardly relative thereto,the air pressure in the cylinder and piston means is reduced. When thedrilling operator has balanced the hook load which includes the weightof the drilling string, less the desired bit pressure, with the airpressure in the cylinder and piston means and when the inclined cylinderand piston means have been so arranged that with respect to the relativemovement of the crown block sheave means and framework means, thevertical force component exerted by the inclined cylinder and pistonmeans during variation of the angle of inclination substantiallybalances the weight variation in hook load and therefore the crown blocksheave assembly is maintained in virtually stable fixed relation withrespect to the sea bed.

This action of the inclined cylinder and piston means supporting thecrown block sheave assembly may be best understood by referring to FIG.13 which shows several curves indicating the percentage change of hookload variation with respect to vertical displacement of the crown blocksheave assembly. In FIG. 13 the abscissa represents a stroke of thecrown block sheave assembly for a distance of 15 feet. At the center ofthe 15 foot stroke, is provided an ordinate showing the percent of hookload variation on down stroke and up stroke of the crown block sheaveassembly. It will be understood that in setting up this system thedrilling operator balances the hook load at approximately the centerportion of travel of the sheave assembly. The exponential line indicatedat A represents a characteristic curve of vertically positionedpneumatic cylinder and piston means wherein the slope of the curve is afunction of the ratio of cylinder displacement volume to accumulatorvolume, a curve being determined by pressure-volume gas formula PV"=Cwherein P is the gas pressure, V is the volume, K is the constant and Cis the cylinder force which is to be constant. It will be noted thatover the 15 foot stroke the variation of hook load is approximately 17percent which is excessive hook load variation for standard drillingpractices.

Curve B (no trim) represents a condition where the cylinder and pistonmeans are inclined and have a constant gas pressure or force, that is,an infinite accumulator volume, to resist movement of the crown blocksheave means. Thus the vertical force component on the sheave meansdecreases as the cylinder and piston means changes its angle ofinclination as the crown block sheave assembly moves to the lowerportion of its stroke. Curve C is representative of a compensatingsystem utilizing the resilient support means described hereinabove. Thusas the sheave means moves from the midpoint of its stroke, downwardlytoward the bottom ofits stroke (when the vessel heaves upwardly) the percent of hook load variation is approximately only 0.80 percent. As thesheave means moves from the midpoint of its stroke upwardly (as when theheave motion of the vessel is downwardly) the percent variation of thehook load at the top of the stroke is only about 4 percent of the totalload variation. Thus over the entire stroke range of 15 feet, the totalload variation is slightly less than percent. However, during the footstroking range as measured from the bottom of the stroke to a few feetabove the midpoint of the stroke which may normally be the working rangeof the crown block sheave assembly, since the drilling bit movesdownwardly during the drilling, the total load variation is less than 2percent, that is plus 0.80 percent and minus 1.20 percent. Thus a narrowband of hook load variation is achieved and a nearly constant bit weightis maintained during drilling. As the crown block sheave assembly movesvertically, the total cylinder force may be represented by the formulaPl=Po (Vo/Vl and the vertical component changes as Fv=Fc sin 0. Bybalancing these force changes including the design parameters of thesize of the cylinder and piston means, the angular variation of thecylinder and piston means during the stroke of the sheave means and gasaccumulator volume, a total change in the vertical force in the order of5 percent of the hook load excluding frictional losses may be achieved.Moreover, the inclined gas cylinder and piston means which serves as themain support for the crown block sheave assembly has reduced the gasvolume requirements for an example as given above, to only 100 cubicfeet and thus it becomes practical to mount working accumulatorsadjacent the cylinder and piston means on the frame work means 22 atplatform 23. Thus no large gas conducting lines must be run up thederrick legs and friction losses in the lineare minimized.

The load displacement relationship is also effected by other parameterssuch as friction in the system and fluid line losses. In addition, thekinematics of the idler sheave mechanism also affects the loaddisplacement relation and for the idler sheave support and linkagearrangement previously described, an exemplary load displacement curveis shown in curve D. In curve D total load variation over the fullstroke of 15 feet is indicated at about 4 percent. For a specificdrilling situation, the idler sheave linkage configuration may bemodified to virtually eliminate any change in hook load by providing arelatively flat curve over a major portion of the stroke of a sheaveassembly.

During operation as the drilling hole depth is increased, the crownblock sheave assembly will stroke lower and lower in thevertical guideframework. The drilling operator may then pay out more of line 26 inorder to bring the crown block stroke into the desired range andposition with respect to the vertical guide framework.

In the event there should be a failure in the gas pressure system, thedashpot cylinder and piston means may be locked out, that is the controlvalves therefore, may be closed so that the liquid in the dashpot systemwill serve as a cushion and will prevent damage to the drillingequipment. Under such conditions the compensating system is inoperativeand drilling may be continued as if no compensation were provided.

It will be understood that while in the examples of the invention shownin FIGS. 4-7, the dashpot means are shown in vertical position anddirectly beneath the sheave shaft of the crown block sheave assembly, itmay be desirable to locate separate dashpot cylinder and piston means ina different position in order to permit, for example, vertical piperacking which may require utilization of the space immediately adjacentto the vertical pathway of the crown block sheave assembly. Under suchconditions the vertical guide rails or columns may be set further apartand the dashpot means arranged to act upon the carriage of the crownblock sheave assembly. Of course in the example'of the invention shownin FIGS. 14 and 15, the dashpot func-' tions are provided by theconstruction of the main cylinder and piston means 178 and by use of theshock absorbing means 214 and 215.

The above-described crown block motion compensating system lends itselfto an even further reduction of hook load variation verses relativedisplacement of the sheave assembly by first utilizing hydrauliccylinders to positively drive the crown block sheave assembly such asfor example shown in FIG. 8. Since virtually all of the load isresiliently supported upon the air actuated cylinder and piston means,only the relatively small unbalanced load must be driven. Thus powerrequirements for such auxiliary hydraulic cylinders are relativelysmall. Such liquid cylinder and piston means may be driven and maderesponsive to the changes in tension or vertical motion as indicated bythe vertical motion sensing means previously described.

When the positive mechanical link is provided be tween the crown blocksheave assembly and the sea bed or riser pipe as described in FIGS. 9,10, and 10a virtually no bit weight variation is permitted and thusdrilling operational efficiencies are greatly increased.

Such vertical sensing systems as described in FIGS. 9, l0, and afacilitate very precise operations such as placing the blowout preventerstack, logging, setting packers, or fishing because the crown blocksheave assembly is essentially fixed relative to the sea floor so thatsuch conditions virtually simulate those of offshore drill platforms offixed leg type.

It will be understood that various changes and modifications may be madein the embodiment of the invention shown above and all such changeswhich come within the spirit of this invention and which come within thescope of the appended claims are embraced thereby.

We claim:

1. In a motion compensated crown block system for a drilling rig on avessel floating above a sea bed, the combination of:

a sheave block assembly including a crown block sheave and a travelingblock sheave interconnected by a sheave cable, said traveling blockbeing adapted to be connected to a rotary drill string and drill bit;

a framework means on said rig;

guide means on said framework means for vertical movement of said crownblock sheave along a path of selected length above said drill string tocompensate for vertical movement of the vessel relative to the sea bedand drill string;

means for supporting said crown block sheave along said path tocompensate for such relative movement of said vessel,

said supporting means comprising upwardly inclined cylinder and pistonmeans on opposite sides of said path and having upper ends pivotallyconnected to said crown block sheave and lower portions pivotallyconnected to said framework means;

a pressure fluid supply source in communication with said cylinder andpiston means to provide fluid pressure forces at said cylinder andpiston means to support said crown block sheave and the load carriedthereby;

said supply source including accumulator means for said pressure fluidto permit variations in pressure force;

the angle of inclination of said cylinder and piston means beingvariable between ends of the path of travel of the crown block sheave toprovide change in vertical force components of the cylinder and pistonmeans in proportion to the change in pressure fluid support forces ofthe accumulator means whereby said support force and vertical forcecomponent are compensated and substantially uniform loading ismaintained at said crown block sheave; and means for sensing motion ofsaid crown block sheave relative to the sea bed and operably connectedto the crown block sheave for regulating the fluid pressure of saidpressure fluid supply source. 2. In a system as stated in claim 1wherein said accumulator means is mounted at the water table and isreduced in volume because of said force compensation. 3. In a system asstated in claim 1 wherein said means for sensing motion of said crownblock sheave relative to the sea bed comprises a tension line meansconnected at one end to said water table, a pulley attached to saidcrown block sheave over which said tension line passes, a pulley at thesea floor through which said line passes, and the opposite end of saidtension line being connected to a draw works on said vessel.

4. In a system as stated in claim 1 wherein said means for sensingmotion of said crown block sheave relative to the sea bed includes atension line having one end connected to said crown block sheave,

the opposite end of said line having a connection to a riser pipe on thepipe string,

and means on the vessel for adjusting tension of the tension line means.

5. In a system as stated in claim 1 wherein said means for sensingmotion of said crown block sheave relative to the sea bed includes atension line means connected to said crown block sheave,

said tension line means being also connected to a guide line connectedto the sea floor,

andmeans onsaid vessel for adjusting the tension of said tension linemeans.

6. In a system as stated in claim 1 wherein each cylinder and pistonmeans includes a cylinder member closed at one end,

and a solid piston member occupying a major portion of the volume of afluid chamber on the same side of said piston head as said pistonmember.

7. In a system as stated in claim 1 wherein each cylinder and pistonmeans includes a cylinder member closed at one end,

a hollow piston member,

a piston head on said piston member including passageways incommunication with opposite sides of said piston head,

and valve means in said piston head.

8. In an apparatus for minimizing variations in loading of a sheaveblock means where load variations occur through relative movement of afloating vessel and the sheave block means, the combination of:

a sheave block assembly including a crown block, a

traveling block, and a sheave cable reaved therebetween and having adead line fastened to said vessel and a fast line connected to drawworks, said traveling block being adapted to be connected to a rotarydrill string;

support means for said sheave block assembly including upwardly inclinedcylinder and cooperable piston members mounted for pivotal movement fromsaid vessel and pivotally connected to. the sheave block assembly toimpart an upwardly directed load supporting force to said sheave blockassem-' a fluid pressure supply source in fluid communication with saidcylinder member for exerting fluid pres sure on said piston member toprovide said load supporting force;

said sheave block assembly being limited to movement along a stroke pathof selected length;

said load supporting force changing as said cylinder member and pistonmember move relative to each other during relative motion of said sheaveblock assembly and said floating vessel;

the angle of inclination of said cylinder and piston members changingduring relative motion of the sheave block assembly and floating vesseland causing a change in vertical force component exerted by saidcylinder and piston members on the sheave block assembly,

the changes in vertical force components being virtually compensated bythe change in load supporting forces of the fluid pressure supply sourcewhereby over a path length of normal stroke the percentage of loadvariation in respect of hook load at the sheave block assembly is in theorder of about 2 percent of the supported load.

9. In a system as stated in claim 8 wherein each cylinder and pistonmeans includes a cylinder member closed at one end,

a hollow piston member having a chamber in communication with thecylinder member,

and port means in said hollow piston member for bypassing pressure fluidaround the piston head of the piston member.

10. In a system as stated in claim 8 including a pair of idle sheavessupported from said framework laterally of said sheave means;

a link means pivotally interconnecting said crown block sheave and eachidle sheave,

a link member pivotally connected to said idle sheave and to saidframework means,

said link means and member defining a virtually laterally horizontallydirected path for each idle sheave with respect to movement of saidcrown block sheave means.

11. In a system as stated in claim 8 including top and bottom dashpotmeans supported in said framework means at opposite ends of the path oftravel of said sheave means.

12. In a system as stated in claim 8 including trim cylinder and pistonmeans on said framework means for further minimizing load variations atthe crown block sheave comprising a trim cylinder and piston having oneend connected to said crown block sheave at the pivotal connection ofthe upper end of the support cylinder and piston means and having itsother end connected to said framework means at a location between theends of the path of travel of the crown block sheave. 13. In anapparatus stated in claim 8 wherein the load variation in respect ofbook load of the load supported by the sheave block means is betweenabout +0.80 and -l.20 percent.

14. In an apparatus as stated in claim 8 wherein over the path length ofmaximum stroke of the sheave block assembly the percentage change ofload variation in respect of hook load at the sheave block means is inthe order of 5 percent or less.

15. In an apparatus as stated in claim 8 wherein said fluid pressuresupply source includes accumulator means of minimal volume mountedadjacent the cylinder members.

16. In an apparatus as stated in claim 8 wherein said support means forsaid sheave block assembly also includes trim cylinder and pistonmembers connected with said sheave block assembly and operable betweenends of the stroke path and movable into upwardly and downwardlyinclined relation to said sheave block assembly.

17. In an apparatus as stated in claim 8 including means for sensingmotion of said sheave block assembly comprising a line of fixed lengthextending between said sheave block assembly and a reference connectionpoint relatively immovable with respect to said floating vessel.

18. In an apparatus as stated in claim 8 wherein said fluid pressuresupply source includes a line in fluid communication between said supplysource and said cylinder member;

and valve means in said line for controlling the rate of movement ofsaid piston member upon rapid change in loading of the sheave blockmeans.

19. In an apparatus as stated in claim 8 including dashpot meanssupported at opposite ends of the path of travel of said sheave blockassembly for cushioning movement of the crown block sheave at ends ofits path of travel.

1. In a motion compensated crown block system for a drilling rig on avessel floating above a sea bed, the combination of: a sheave blockassembly including a crown block sheave and a traveling block sheaveinterconnected by a sheave cable, said traveling block being adapted tobe connected to a rotary drill string and drill bit; a framework meanson said rig; guide means on said framework means for vertical movementof said crown block sheave along a path of selected length above saiddrill string to compensate for vertical movement of the vessel relativeto the sea bed and drill string; means for supporting said crown blocksheave along said path to compensate for such relative movement of saidvessel, said supporting means comprising upwardly inclined cylinder andpiston means on opposite sides of said path and having upper endspivotally connected to said crown block sheave and lower portionspivotally connected to said framework means; a pressure fluid supplysource in communication with said cylinder and piston means to providefluid pressure forces at said cylinder and piston means to support saidcrown block sheave and the load carried thereby; said supply sourceincluding accumulator means for said pressure fluid to permit variationsin pressure force; the angle of inclination of said cylinder and pistonmeans being variable between ends of the path of travel oF the crownblock sheave to provide change in vertical force components of thecylinder and piston means in proportion to the change in pressure fluidsupport forces of the accumulator means whereby said support force andvertical force component are compensated and substantially uniformloading is maintained at said crown block sheave; and means for sensingmotion of said crown block sheave relative to the sea bed and operablyconnected to the crown block sheave for regulating the fluid pressure ofsaid pressure fluid supply source.
 2. In a system as stated in claim 1wherein said accumulator means is mounted at the water table and isreduced in volume because of said force compensation.
 3. In a system asstated in claim 1 wherein said means for sensing motion of said crownblock sheave relative to the sea bed comprises a tension line meansconnected at one end to said water table, a pulley attached to saidcrown block sheave over which said tension line passes, a pulley at thesea floor through which said line passes, and the opposite end of saidtension line being connected to a draw works on said vessel.
 4. In asystem as stated in claim 1 wherein said means for sensing motion ofsaid crown block sheave relative to the sea bed includes a tension linehaving one end connected to said crown block sheave, the opposite end ofsaid line having a connection to a riser pipe on the pipe string, andmeans on the vessel for adjusting tension of the tension line means. 5.In a system as stated in claim 1 wherein said means for sensing motionof said crown block sheave relative to the sea bed includes a tensionline means connected to said crown block sheave, said tension line meansbeing also connected to a guide line connected to the sea floor, andmeans on said vessel for adjusting the tension of said tension linemeans.
 6. In a system as stated in claim 1 wherein each cylinder andpiston means includes a cylinder member closed at one end, and a solidpiston member occupying a major portion of the volume of a fluid chamberon the same side of said piston head as said piston member.
 7. In asystem as stated in claim 1 wherein each cylinder and piston meansincludes a cylinder member closed at one end, a hollow piston member, apiston head on said piston member including passageways in communicationwith opposite sides of said piston head, and valve means in said pistonhead.
 8. In an apparatus for minimizing variations in loading of asheave block means where load variations occur through relative movementof a floating vessel and the sheave block means, the combination of: asheave block assembly including a crown block, a traveling block, and asheave cable reaved therebetween and having a dead line fastened to saidvessel and a fast line connected to draw works, said traveling blockbeing adapted to be connected to a rotary drill string; support meansfor said sheave block assembly including upwardly inclined cylinder andcooperable piston members mounted for pivotal movement from said vesseland pivotally connected to the sheave block assembly to impart anupwardly directed load supporting force to said sheave block assembly; afluid pressure supply source in fluid communication with said cylindermember for exerting fluid pressure on said piston member to provide saidload supporting force; said sheave block assembly being limited tomovement along a stroke path of selected length; said load supportingforce changing as said cylinder member and piston member move relativeto each other during relative motion of said sheave block assembly andsaid floating vessel; the angle of inclination of said cylinder andpiston members changing during relative motion of the sheave blockassembly and floating vessel and causing a change in vertical forcecomponent exerted by said cylinder and piston members on the sheaveblock assembly, the changes in vertical force components Being virtuallycompensated by the change in load supporting forces of the fluidpressure supply source whereby over a path length of normal stroke thepercentage of load variation in respect of hook load at the sheave blockassembly is in the order of about 2 percent of the supported load.
 9. Ina system as stated in claim 8 wherein each cylinder and piston meansincludes a cylinder member closed at one end, a hollow piston memberhaving a chamber in communication with the cylinder member, and portmeans in said hollow piston member for bypassing pressure fluid aroundthe piston head of the piston member.
 10. In a system as stated in claim8 including a pair of idle sheaves supported from said frameworklaterally of said sheave means; a link means pivotally interconnectingsaid crown block sheave and each idle sheave, a link member pivotallyconnected to said idle sheave and to said framework means, said linkmeans and member defining a virtually laterally horizontally directedpath for each idle sheave with respect to movement of said crown blocksheave means.
 11. In a system as stated in claim 8 including top andbottom dashpot means supported in said framework means at opposite endsof the path of travel of said sheave means.
 12. In a system as stated inclaim 8 including trim cylinder and piston means on said framework meansfor further minimizing load variations at the crown block sheavecomprising a trim cylinder and piston having one end connected to saidcrown block sheave at the pivotal connection of the upper end of thesupport cylinder and piston means and having its other end connected tosaid framework means at a location between the ends of the path oftravel of the crown block sheave.
 13. In an apparatus as stated in claim8 wherein the load variation in respect of hook load of the loadsupported by the sheave block means is between about +0.80 and -1.20percent.
 14. In an apparatus as stated in claim 8 wherein over the pathlength of maximum stroke of the sheave block assembly the percentagechange of load variation in respect of hook load at the sheave blockmeans is in the order of 5 percent or less.
 15. In an apparatus asstated in claim 8 wherein said fluid pressure supply source includesaccumulator means of minimal volume mounted adjacent the cylindermembers.
 16. In an apparatus as stated in claim 8 wherein said supportmeans for said sheave block assembly also includes trim cylinder andpiston members connected with said sheave block assembly and operablebetween ends of the stroke path and movable into upwardly and downwardlyinclined relation to said sheave block assembly.
 17. In an apparatus asstated in claim 8 including means for sensing motion of said sheaveblock assembly comprising a line of fixed length extending between saidsheave block assembly and a reference connection point relativelyimmovable with respect to said floating vessel.
 18. In an apparatus asstated in claim 8 wherein said fluid pressure supply source includes aline in fluid communication between said supply source and said cylindermember; and valve means in said line for controlling the rate ofmovement of said piston member upon rapid change in loading of thesheave block means.
 19. In an apparatus as stated in claim 8 includingdashpot means supported at opposite ends of the path of travel of saidsheave block assembly for cushioning movement of the crown block sheaveat ends of its path of travel.